Aluminium, Tau and Alzheimer's Disease

Abstract

I would like to bring to the attention of your readers fundamental flaws in both the methodology and philosophy of a paper recently published in JAD [13]. The most significant problems concern the former and they seriously undermine both the interpretation of the results and the conclusions drawn by the authors. For example, Fig. 1 purports to demonstrate the effects of AlCl3 on tau aggregation in vitro. What these experiments actually demonstrate is that AlCl3 inhibited the heparin-induced aggregation of tau. When one considers the extraordinarily high concentrations of AlCl3 used in these experiments (0.2–20.0 mM) compared to those of tau (10 μM) and heparin (10 μM) and the well known ability for aluminium hydroxide to adsorb and precipitate the highly polyanionic heparin [3,16,17], (it is not a coincidence that aluminium is a major contaminant of parenteral heparin solutions [2,9]) it should have come as no surprise to find that super-saturated solutions of AlCl3 precipitated heparin and inhibited its induction of the assembly of tau filaments in vitro. However, to test the influence of aluminium upon heparininduced aggregation of tau the authors needed to ensure that heparin was present to a significant excess, at least ten times the concentration of AlCl3. There can be no good reason for designing experiments in which the ratio of total aluminium to tau was at its minimum 20 and at its maximum 2000! There can be no physiological significance to such experiments and I urge the authors to repeat these experiments in the presence of a significant excess of heparin (or another inducer of tau polymerisation which will not bind aluminium) and using stoichiometric ratio’s of aluminium to tau of no more than 10. Only then will the authors actually address the hypothesis that they set out to test. Unfortunately this was not the only significant oversight in the chosen methods. The remainder of the experiments use aluminium which the authors describe as aluminium maltolate. However, their method of preparing Almaltolate will not result in Al-maltolate and nor do they provide any evidence that what they have in their stock solutions is Al-maltolate. The decision to prepare their maltol solution in phosphate-buffered saline (PBS) ensured that they introduced a minimum of 5 mM inorganic phosphate into the Al-maltolate stocks and this would have ensured that a significant proportion of the total aluminium would have been precipitated at pH 7.4 as mixed hydroxyphosphates. In addition if the dilutions of this stock solution, used in particular for the experiments reported in Fig. 2, were made with PBS (which includes 10 mM inorganic phosphate), as it appeared they were, then this would have ensured that approximately all of the aluminium would be precipitated as aluminium hydroxyphosphates in these treatments. Thus the N2a cells were not exposed to the designated concentrations of Al-maltolate but to unspecified concentrations of aluminium hydroxyphosphates in the presence of maltol. These problems will also influence the intraperitoneal injections of aluminium reported in Table 1 and Fig. 3 where one is also left wondering how you can produce a 50 mM Al-maltolate stock from an original stock which was only 25 mM? One final word of caution concerning experiments with aluminium and tau is that aluminium catalyses both phosphorylation [12] and phosphoincorporation [1] of proteins, including tau, and considering the role played by phosphorylation in the aggregation of tau one